pcnt_unit_stop() function

- ESP_OK: Stop PCNT unit successfully - ESP_ERR_INVALID_ARG: Stop PCNT unit failed because of invalid argument - ESP_ERR_INVALID_STATE: Stop PCNT unit failed because the unit is not enabled yet - ESP_FAIL: Stop PCNT unit failed because of other error

This function should be called when the unit is in the enable state (i.e. after calling `pcnt_unit_enable()`) The stop operation won't clear the counter. Also see `pcnt_unit_clear_count()` for how to clear pulse count value. This function is allowed to run within ISR context This function will be placed into IRAM if `CONFIG_PCNT_CTRL_FUNC_IN_IRAM`, so that it is allowed to be executed when Cache is disabled